Optical deflection units for targeted radiation in scanning, ophthalmological measuring and therapy systems and other applications, produce radiation, e.g., through laser or superluminescent diodes, utilized for targeted illumination of eye structures in order to gain information from the backscattered light portions in the ophthalmological measuring devices or to achieve treatment effects in the eye tissue with therapy systems. From DE 196 35 998 C1, the combination of such a measuring and therapy device using targeted optical radiation is known.
Examples of ophthalmological measuring devices are biometric systems for the determination of sizes, distances, and/or geometric relations of eye structures. Such measurement results are required for the adjustment of implants, such as intraocular lenses after cataract operations, but are also suited for the diagnosis of certain clinical pictures, e.g., narrow-angle glaucoma.
Therapy systems using specifically applied, targeted radiation are, for example, laser systems for follow-up treatment of cataract diseases or for coagulation of retinal areas in case of retinal detachments.
For scanning measuring as well as therapy devices, one- or two-dimensional deflection patterns are most commonly used, which are to be realized by the deflection unit. Examples are laser retina treatments, whereby adjusted dot patterns for different retinal areas are frequently used in order to achieve optimal therapy effects at minimized patient impairment. This also applies to imaging, ophthalmological measuring systems such as optical coherence tomographs (OCT) or scanning laser ophthalmoscopes (SLO).
Such imaging, ophthalmological measuring systems are primarily used to produce two-dimensional images, sectional images, and volume scans of various areas of the eye and to evaluate with regard to visual impressions, sizes, and distances of certain eye structures. For example, solutions are known from prior art, which thereto utilize optically based scan systems.
Measurements continue to be performed with biometric, ophthalmological measuring systems, whereby a measurement beam is positioned but otherwise not deflected. An example thereto is the sectional measurement on the eye, according to US 2005/018137 A1. However, if position and curvature of boundary layers and structures, which are not only located on the optical axis of the eye, are to be detected biometrically, two-dimensional measurements are also used (according to WO 2006/015717 A1).
A first group of the imaging, ophthalmological measuring systems are hereto tomography systems, which, e.g., are based on the so-called OCT (optical coherence tomography) method, whereby short coherent light with the help of an interferometer is employed for distance measurement of reflective and scattering materials (US 2007/0291277 A1). Through depth scans, the optical coherence tomography on the human eye delivers measurable signal responses due to the scattering at index of refraction changes, particularly at optical boundaries. Known variations are the time-domain OCT (TD-OCT) and Fourier-domain OCT (FD-OCT) with its versions, which are based on the application of spectrometers (SD-OCT) of spectrally tunable light sources (SS-OCT). Thereto, see Leitgeb et al. “Performance of Fourier domain vs. time domain optical coherence tomography,” Opt. Express Vol. 11, 889 (2003).
A second group comprises ophthalmoscopes, particularly scanning laser ophthalmoscopes (SLO), which, in addition to the OCT-based tomography systems, also represent known and important tools for diagnosis and therapy in ophthalmology (US 2005/030475 A1).
In order to obtain the images required for the diagnosis in the form of two-dimensional images, sectional images, and volume scans, scans, in addition to A-scans (individual depth profiles), are required transversally in a first (B-scans) and a second direction (volume scans and scans at a constant reference plane, so-called C-scans).
Thereby it is very important to record those scans very quickly since the available attention span of a patient (with barely 2 sec.) is very limited. Furthermore, according to Yun et al. “Motion artifacts in optical coherence tomography with frequency-domain ranging,” Opt. Express, Vol 12, 2977 (2004), motion artifacts can, as a result, be limited and corrected in accordance with the solution in WO 2006/077107 A1.
Therefore, very quick deflection systems for the measurement beams must be deployed in such imaging systems. At the same time, said deflection systems must be able to render a predetermined scan pattern very accurately, linearly, and very reproducibly, ensuring that the emerging sectional images and volume scans exhibit no distortions which would make the evaluation of the structures needlessly difficult.
Said great challenges to the deflection devices regarding speed and control accuracy are, e.g., further enhanced when so-called tracking systems, already widely used in ophthalmology, are utilized which detect, register and/or actively compensate the eye movements during the course of the measurements. Such tracking systems are described, for example, in U.S. Pat. No. 6,726,325 B2; U.S. Pat. No. 7,365,856 B2; and US 2006/228011 A1.
In order to meet these requirements regarding speed and control accuracy, stably rotating polygon mirrors for realizing close to linear sawtooth or triangular scans, or galvanometer mirrors within closed control loops are used according to prior art.
Polygon mirrors are able to scan very quickly and stably but are limited to a defined deflection pattern in a defined direction. Additionally, they have distortion and are expensive.
By contrast, galvanometer mirrors are able to realize different scan patterns but also require great electronic control efforts (closed-loop galvanometers with position feedback sensors) in order to reproduce a predetermined deflection pattern with accuracy, linearity, and reproducibility acceptable for imaging.
Therefore, combinations of both systems are also frequently employed as scan unit in ophthalmological devices, whereby a quickly rotating polygon mirror produces a fixed deflection pattern in a deflection direction and a galvanometer mirror realizes a possibly deviating deflection pattern in a second, slower deflection direction.
The most frequently utilized deflection systems in ophthalmological scanners, as described in US 2008/231808 A1, exhibit modern galvanometer scanners with an optical position detection system, which, via an electronic control unit, allows for active control of the mirror movement, including the damping of interferences (U.S. Pat. No. 5,999,302 A).
With the polygon mirror, a stabilization of the rotation frequency is essentially realized. Major disadvantages of galvanometer scanners and polygon mirrors are the limitation of one rotation axis each as well as wear and tear and required lubrication of the bearings.
The solutions for deflection systems for scanning measurement value logging, which are known in accordance with prior art and described herein, particularly fast volume scans, are “oversized” for the measuring and therapy tasks to be solved and have the additional disadvantage of being very elaborate and expensive.
Aside from galvanometer and polygon scanners, additional optical systems for deflection of light beams are known, according to prior art, which do not apply to the field of ophthalmology.
Examples of acousto-optical and electro-optical scanners are known from U.S. Pat. No. 7,050,208 B2 and US 2009/0073538 A1. Even though they are suited as optical deflection systems without mechanical moment of inertia for very fast deflections, they also require great efforts regarding control since very exact electrical high voltages or high-frequency signals are generated.
DE 38 33 260 A1 describes a light deflection device, whereby a mirror is moved as quickly and precisely as possible around a rotation axis located in the mirror plane. Thereto, the suspension of the mirror is disclosed as flexural pivot, the spring elements of which consist of piezoelectric material. Through applying voltage to the spring elements, the mirror can be moved with this mechanism at low energy and with high motion frequency due to the piezoelectric material properties. Piezoelectric actuators also exhibit the disadvantage of generating very exact high voltages. Furthermore, it is known that this type of control with piezoelectric actuators leads to premature material fatigue due to small microcracks.
A support for a scanning mirror is disclosed in DE 29 51 593 A1. Thereby, the scanning mirror oscillates with natural resonance around an axis in the mirror plane. In order to facilitate a larger rotation angle of the oscillation at the same oscillation frequency, the support is formed from two aligned connected flexural pivots. Hereto it is disadvantageous that deflections can only be realized at certain deflection frequencies which does not allow for high flexibility regarding the deflection pattern generation.
The invention herein is based on the task of developing an optical deflection unit for ophthalmological diagnosis and therapy systems, whereby the known disadvantages from prior art are remedied and which is significantly more cost-effective and less elaborate. Furthermore, the optical deflection unit shall be characterized by simplicity, durability, robustness, improved reproducibility as well as decreased aging and temperature effects. In addition, the optical deflection unit should be capable of realizing relatively large deflections of ±10° in two deflection dimensions with a compact design.